[Linux 常見問題] Shell - How to calculate time difference in bash script?

Source From Here
Question
I print the start and end time using date +"%T", which results in something like:

10:33:56
10:36:10

How could I calculate and print the difference between these two? I would like to get something like:

2m 14s

How-To
I guess the easiest solution would be to obtain the time as the number of seconds since the Unix epoch, and then subtract them, doing the time arithmetic before displaying.

The -u parameter instructs date to return the time in UTC. This is needed in order to keep the timeline continuous : without that option, the calculation of diff could return an incorrect value if a DST transition happened between the two calls to date. This parameter is required by POSIX, so I bet it's widely supported across various Unices.
- test.sh

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1. #!/bin/sh  
2. date1=$(date -u +"%s")  
3. sleep 5  
4. date2=$(date -u +"%s")  
5. diff=$(($date2-$date1))  
6. echo "$(($diff / 60)) minutes and $(($diff % 60)) seconds elapsed."  

Execution result:

# ./test.sh
0 minutes and 5 seconds elapsed.